Power supplies (power converters) are often contained in electrical devices to convert alternating current (AC) power received from, for example, a conventional wall outlet to a desired direct current (DC) power for the device. The AC power has continuously varying voltage and the DC power provided to the device is typically at a constant voltage.
FIG. 1 illustrates a high level block diagram of an example power supply 100 typically used to convert AC power to DC power with a constant voltage. The power supply 100 includes a transformer 110, a rectifier (full-wave rectifier) 120, and a storage device (shunt capacitor) 130. The transformer 110 is to adjust (step up, step down) the input AC voltage to the desired voltage for a particular device. For example, the transformer 110 may step down 120 V received (e.g., from a wall outlet) to 60 V. It is possible that a device's voltage requirement could be the same as the incoming AC voltage in which case the transformer 110 could be eliminated.
The full wave rectifier 120 converts the AC voltage to a DC voltage by only providing the voltage above a certain reference voltage or ground (positive portion of the voltage). The DC voltage from the full wave rectifier 120 still varies from the reference voltage (e.g., 0 V) to a peak voltage (associated with the desired voltage of a device). The DC voltage is provided to and utilized to charge the shunt capacitor 130. The charged shunt capacitor 130 is to provide the required power (current needed at substantially constant desired voltage) to a load 140. In order to provide the required power to the load 140, the shunt capacitor 130 has to be capable of holding the majority of its charge for a relatively long period of time and thus has a relatively high capacitance value.
When the AC voltage provided to the full wave rectifier 120 from the transformer 110 is below the voltage stored in the shunt capacitor 130, the full wave rectifier 120 blocks current flow therethrough. As the voltage stored in the shunt capacitor 130 is drained by the load 140 and falls below the voltage provided by the full wave rectifier 120, the full wave rectifier 120 allows current to pass and thus recharge the shunt capacitor 130. The current provided by the transformer 110 (as controlled by the full wave rectifier 120) is only utilized for a relatively small portion of time to recharge the shunt capacitor 130.
FIG. 2A illustrates a circuit diagram of one common type of power supply 200. The power supply 200 includes the transformer 110, the full-wave rectifier 120, and the shunt capacitor 130. The transformer 110 includes a primary winding 212 and a secondary winding 214, where the adjustment to the incoming AC voltage is based on the ratio of primary windings 212 to secondary windings 214. The secondary windings 214 include an upper tap (T1) 216 and a lower tap (T2) 218. The voltage provided by the taps 216, 218 may be complimentary to each other with respect to a reference voltage (e.g., ground). That is, while T1 is providing a voltage above the reference voltage (positive voltage) T2 is providing a corresponding voltage below the reference voltage (negative voltage) and vice versa.
The full-wave rectifier 120 includes four (4) diodes 222, 224, 226, 228 arranged as a diode bridge. The diodes 222, 224 are connected between the taps 216, 218 and the positive terminal of the capacitor 130. The anodes (positive side) of the diodes 222, 224 are connected to the taps 216, 218 and the cathodes (negative side) are connected to the capacitor 130. The diodes 226, 228 are connected between the taps 216, 218 and the negative terminal of the capacitor 130 (as well as a common point 250 having a reference voltage (e.g., ground) and the common/negative terminal of the load 140). For ease of explanation we will simply refer to the common point 250 having a reference voltage as ground (e.g., 0V). The cathodes of the diodes 222, 224 are connected to the taps 216, 218 and the anodes are connected to the capacitor 130 and ground 250. The diodes 222, 224, 226, 228 allow current to flow in only one direction.
Accordingly, the full wave rectifier 130 provides only the portion of the voltage above ground (positive voltage) from each tap 216, 218 as one output and ground (0V) as the other output. The output of the full-wave rectifier 120 is utilized to charge the shunt capacitor 130. As noted above the charge stored in the shunt capacitor 130 may result in the full-wave rectifier 120 only utilizing current from the transformer 110 a small portion of the time.
The voltage provided by the shunt capacitor 130 may have ripples (vary above and below the desired voltage). Accordingly, the power supply 200 may include a voltage regulator 260 to smooth out the ripples. Alternatively, the voltage regulator may be utilized external to the power supply 200.
FIG. 2B illustrates a circuit diagram of one common type of power supply 205. The transformer 110 includes an additional center tap 219. The center tap 219 will be the cross over point between the upper and lower taps 216, 218, the reference voltage (e.g., 0 V) by which the voltages provided by the taps 216, 218 are measured. The center tap 219 is connected to the negative terminal of the capacitor 130 (as well as the common/negative terminal of the load 140 and possibly ground 250). Accordingly, the full-wave rectifier 120 only needs two (2) diodes 222, 224. The diodes 222, 224 are connected between the taps 216, 218 and the positive terminal of the capacitor 130. The anodes of the diodes 222, 224 are connected to the taps 216, 218 and the cathodes are connected to the capacitor 130.
FIG. 3 illustrates an example timing diagram for the operation of a typical power supply (e.g., 100, 200, 205). Diagram (a) illustrates an example output of the transformer 110. When tap T1 216 is providing a voltage above a reference voltage that is provided by the center tap 219 or connectivity to a common point 250 (positive voltage), tap T2 218 is providing a voltage below the reference voltage (negative voltage) and vice versa. It should be noted that the reference voltage is illustrated as 0V (e.g., ground). Diagram (b) illustrates an example output of the full-wave rectifier 120 assuming there is no voltage stored in the shunt capacitor 130. When tap T1216 is providing a positive voltage the full-wave rectifier 120 provides that voltage as an output and when tap T2 218 is providing a positive voltage the full-wave rectifier 130 provides that voltage as an output.
Diagram (c) illustrates an example charge status of the shunt capacitor 130. From time t0 to time t1, the shunt capacitor 130 is charged by the tap T1216. After the shunt capacitor 130 is charged it provides the required power (current needed at substantially constant voltage) to the load 140. As the shunt capacitor 130 provides the power it begins to discharge (as illustrated the shunt capacitor 130 is slowly discharging from time t1 to between time t2 and t3). Once the charge in the capacitor 130 falls below the charge from a particular tap of the transformer 110, the full-wave rectifier 120 allows the current to pass and thus recharge the capacitor 130 (as illustrated the shunt capacitor 130 is being recharged from a point between time t2 and t3 until time t3).
Diagram (d) illustrates an example output of the full-wave rectifier 120 during operation. Between time t0 and t1, the full-wave rectifier 120 allows the transformer 110 (T1 216) to charge the capacitor 130. After the capacitor 130 is charged the full-wave rectifier 120 blocks the current from the transformer 110 until the voltage from the transformer 110 is above the charge stored in the capacitor 130. As illustrated, for a portion of time between time t2 and t3, T2 218 provides the voltage and for a portion of time between t4 and t5, T1216 provides the voltage.
There are several disadvantages to the common power supply (e.g., 100, 200, 205) that utilizes a shunt capacitor 130 to provide power (current required at substantially constant voltage) to the load 140. The shunt capacitor 130 cannot deliver the required current exactly as needed by the load (e.g., an electrical device) and does not respond to changes in the current required by the load in a timely fashion. Rather, the current delivery from such capacitors is inaccurate (“non-linear”), in that sometimes less current than necessary is delivered and other times more current than necessary is delivered, and in that the shunt capacitor is relatively slow to respond to changes in the current required by the load. Therefore, in devices that benefit from a very linear, accurate delivery of current, the use of the usual power supply impairs performance.
Also, typical power supplies require very large capacitance values in the shunt capacitor. This generally forces manufacturers to use electrolytic capacitors, which are noticeably less linear than other types of capacitors. An electrolytic capacitor in a power supply can sometimes be replaced by several smaller-value non-electrolytic capacitors, but this typically greatly increases cost, size and weight.
Attempts have been made to increase the linear current delivery of current power supplies by utilizing higher quality shunt capacitors and/or by utilizing additional shunt capacitors. These attempts can increase the cost of an electrical device and do not fully eliminate the problem.
Another drawback of the typical power supply is that, as a shunt capacitor 130 releases current into an electrical device, the capacitor's power is constantly depleted. Therefore, it periodically requires a “top-up” of power (current) from the AC source. The amount of power (current) from the transformer 110 which is necessary to top-off a shunt capacitor may be greater than, possibly many times greater than, the power (current) required to operate the device (load). The full-wave rectifier 120 may create electronic “noise” in the system as this burst of power (current) from the transformer begins, and when the burst ends. This electronic “noise” can decrease the performance of the electrical device (load).